question for dota 2

i have a question..how can some1 go up levels in dota 2..i mean i 've played so many games didnt earn anything.and some broadcasters in some streams i watch they are 2 lvl or 3 lvl or some level..why is that?can some1 explain it to me why i dont go some levels up??i am still one level with zero exp.why?
and is there any limit to get some exp?i mean for example in every 500 games u get a level.
ty beforehand :)